The process of development of the immune system and the mechanism by which antibody diversity is generated remain areas in which current understanding is very limited. Xenopus laevis is being used in our laboratory as a model system for immune development because of the many advantages of studying development in an amphibian. The onset of immunoglobulin synthesis has been characterized, and properties of the immunoglobulins synthesized first during embryogenesis will be investigated. Central to an understanding of immune development is an understanding of the ways in which immunoglobulin genes are manipulated prior to the onset of immunoglobulin synthesis. A major focus of our research is the isolation of immunoglobulin mRNA sequence. A crude preparation of xenopus immunoglobulin mRNA has been translated in vitro. Sequences corresponding to this mRNA will be inserted into recombinant DNA molecules and clones screened for the presence of immunoglobulin sequences. Purified immunoglobin DNA sequences will then be used as a probe for changes in the number and organization of the immunoglobulin genes during the generation of antibody diversity.